This module responds to
<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
with
<failure xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
,
in order to test how clients and server behave.
With the plugin installer in Prosody 0.12 you can use:
sudo prosodyctl install --server=https://modules.prosody.im/rocks/ mod_tlsfail
For earlier versions see the documentation for installing 3rd party modules